In general, a photographic light-sensitive material comprising a coat of a silver halide emulsion is subject to various kinds of pressure. For example, ordinary photographic negative film is wound on a cartridge, bent when loaded into a camera or pulled when moved from one frame to another.
Further, a sheet film such as printing light-sensitive material and direct medical X-ray-sensitive material is often subject to bending due to handling.
Furthermore, any light-sensitive material is subject to a great pressure upon cutting or processing.
When a photographic light-sensitive material is subject to various kinds of pressure, silver halide grain is subject to pressure via the gelatin (binder) which carries silver halide grain or the plastic film (support). It is known that when silver halide grain is subject to pressure, the photographic light-sensitive material shows some change in photographic properties as reported in detail in K. B. Mather, J. Opt. Soc. Am., 38, 1054 (1948), P. Faelens and P. de Smet, Sci. et Ind Phot., 25, No. 5, 178 (1854) and P. Faelens, J. Phot. Sci., 2, 105 (1954).
Therefore, it is desirable to provide a photographic light-sensitive material which is not susceptible to the effect of pressure upon the photographic properties.
Known methods for improving the pressure characteristics include providing the light-sensitive material with some plasticity from a polymer or emulsion or a method which comprises reducing the proportion of silver halide content to gelatin content in the silver halide emulsion. The following methods are intended to prevent pressure from reaching silver halide grains.
For example, British Patent No. 738,618 discloses a method which comprises using a heterocyclic compound to prevent undesirable changes in photographic properties due to pressure. British Patent No. 738,637 discloses a method which comprises using an alkyl phthalate. British Patent No. 738,639 discloses a method which comprises utilizing an alkyl ester. U.S. Pat. No. 2,960,404 discloses a method which comprises using a polyvalent alcohol. U.S. Pat. No. 3,121,060 discloses a method which comprises using a carboxyalkyl cellulose. JP-A-49-5017 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method which comprises using a paraffin and a carboxylic acid. JP-B-53-28086 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method which comprises using an alkyl acrylate and an organic acid.
However, the above described approach of providing the light-sensitive material with some plasticity is disadvantageous because the addition of an effective amount of these compounds results in a reduction in the mechanical strength of the emulsion layer. Thus, these compounds can only be added in limited amounts. This approach is also disadvantageous in that an increase in the added amount of gelatin causes a reduction in development speed. Thus, the known methods are ineffective.
In general, hexagon, octahedron, potato-shaped or spherical silver halide grain is less susceptible to deformation due to external pressure than tabular silver halide grain having a larger diameter/thickness ratio, because of their structure. Therefore, the above described methods may improve the pressure characteristics to a relatively small degree, but they do not improve pressure characteristics to a satisfactory level.
On the other hand, as the recent demand has grown for silver halide color negative films with a higher sensitivity and a smaller format, it has been keenly desired to provide a high sensitivity color negative photographic light-sensitive material which exhibits excellent picture quality.
In order to meet this requirement, a method which comprises the use of tabular grains for the purpose of improving the sensitivity (including the efficiency of color sensitization by a sensitizing dye), improving the relationship between the sensitivity and the graininess, improving the sharpness and improving the covering power is disclosed in U.S. Pat. Nos. 4,424,226, 4,414,310, 4,433,048, 4,414,306, and 4,459,353. Thus, tabular silver halide grain having a high aspect ratio is advantageous in that its properties are most desirable. However, an experiment showed that when tabular silver halide grain having a high aspect ratio (e.g., 8 or more) is incorporated in a layer other than the farthest light-sensitive layer from the support, particularly a green- or red-sensitive layer, in a so-called forward order layer structure (in the order of a blue-sensitive layer, a green-sensitive layer and a red-sensitive layer from the remote position of the support), the sharpness at a low frequency is deteriorated.
Japanese Patent Application No. 61-311130 discloses tabular silver halide grains having an aspect ratio of 8 or less which are intended to solve the above described problem.
However, tabular silver halide grains are remarkably weak to external forces due to their structure. Therefore, the above described method cannot provide tabular silver halide grains with satisfactory pressure characteristics. Tabular silver halide grains disclosed in Japanese Patent Application No. 61-311130 exhibits fog and an increase in sensitivity when subject to pressure. In order to improve such pressure characteristics, intensive studies have been undertaken. However, an effective approache has not been found.
Regarding the pressure characteristics of the negative portion of a negative paper system, it is known that since the fogged portion in the negative portion has a high density on the paper, the density change in this portion is hardly remarkable. Therefore, the pressure marks in the negative portion of a negative photographic light-sensitive material is of little consequence. However, the sensitization and desensitization, due to pressure in the gradation portion of a negative paper system, causes problems. These problems need to be eliminated.